1. Field of the Invention
The present invention pertains to systems for converting thermal and radiant energy into electrical energy, and more particularly, the invention relates to photothermophotovoltaic conversion systems.
2. Description of the Prior Art
Systems for converting solar energy to electrical energy at significant production levels require relatively high efficiencies to attain the requisite specific power (W/kg or W/cm.sup.2). Different systems of energy conversion have been proposed in the past, in an attempt to achieve this goal.
Photovoltaic (PV) systems are attractive because they are relatively simple in their construction and operation. However, PV cells used in these systems suffer from the inherent drawback of being unable to convert incoming solar radiation which is below the band gap energy of the cells into electrical energy, and hence that part of the incoming solar spectrum is lost to the photovoltaic system.
An alternative system to photovoltaics is generally known by the term "thermophotovoltaics" (TPV), wherein incoming solar radiation is collected and concentrated by mirrors and/or lenses and is directed onto an intermediate radiator (absorber/emitter) which typically is designed so as to approximate a blackbody emitter. The radiator, in turn, radiates energy onto an array of PV cells which convert the radiation above band gap energy into electrical energy, and reflect the portion below band gap energy back to the radiator, thereby utilizing that portion of the energy spectrum to help to keep the radiator hot. TPV conversion efficiencies approaching 50 percent are theoretically possible. However, TPV conversion efficiencies are extremely sensitive to parasitic system losses, such as losses in the focusing optics, heat losses in the emitter cavity, and losses due to poor reflectivity of the PV cell for below band gap energy. Modeling has shown that the latter-mentioned inefficiency, that of poor reflectivity of the PV cell, would be a primary cause of reduced TPV system performance. For example, poor reflectivity along with small losses (of the order of 10 percent) from the other two types of parasitic losses, typically would reduce TPV conversion efficiencies to below 20 percent.
To date, a preferred system for achieving high solar energy conversion efficiencies uses a multiple band gap solar cell. As in the photovoltaic system described above, only those photons with energy greater than the band gap energy of the cell can be converted into usable electrical energy. However, the energy of a photon which is in excess of the band gap energy is lost as heat in the solar cell, and does not contribute to the useful power output Therefore, multiple band gap cells are arranged such that incoming light is first incident on the highest band gap portion, and the remaining below band gap energy is then directed to a lower band gap portion, and so forth.
At present, only double band gap cells have been fabricated, and the fabrication of only double or triple band gap cells appears to be possible in light of the complexity and associated difficulties involved. For example, monolithic multiple band gap cells must be fabricated from III-V components such as (In.sub.1-x Ga.sub.x)As and (Al.sub.1-x Ga.sub.x)As. Tunnel junction interconnects must be fabricated between the different band gap materials to collect all the current that is photogenerated. The complexity of fabrication of this type of cell results in expensive production costs. For a variety of reasons, unexpected difficulties in approaching the theoretical efficiencies of multiple band gap cells have been experienced. To date, no multiple band gap cell has been fabricated with an efficiency exceeding that of the better single band gap PV cells.